Drill bodies of the type generally mentioned above may be divided into two main categories, viz., on one hand, more or less long narrow bodies, which themselves form complete, usable drilling tools, and on the other hand fairly short attachments, which are connected with longer bodies in order to form, together with the same, complete drilling tools. Such a connection may be permanent, the entire drill being discarded after wear of the cutting edges, alternatively semi-permanent, e.g., by soldering, or detachable, the drill body in question forming a wear body or a so-called loose top, which simply may be replaced after wear and allow long-term use of a carrying basic body.
Drills and drill bodies may be divided into different categories also in other respects. Thus, some drills may have a symmetrical geometry by including two or more identical cutting edges that are equi-angularly spaced-apart, while others have an asymmetrical geometry by including a number of cutting inserts that differ from each other in respect of the radial positions thereof and/or the shape thereof. Furthermore, some drills are formed with entirely or partly helicoidal and cross-sectionally concave chip flutes (twist drills), while others again are formed with straight chip flutes that are delimited by plane limiting surfaces.
Common to all types of drills is the axiomatic fact that the individual cutting edge, which provides for chip removal out of the blank or workpiece, is situated in or adjacent to the wall in the front part of the chip flute that forms a rear part surface in the chip flute, as viewed in the direction of rotation of the drill. When the chip is removed from the bottom of the hole to be made, the chip is therefore contained in a hollow space that is delimited by the limiting surface of the chip flute as well as by the cylindrical hole wall generated during the drilling. The purpose of the chip flute is to evacuate the released chip from the hole in the best way while avoiding chip jamming and other disturbing phenomena. For this reason, it is desirable that the chip, as far as possible, is brought to move axially through the chip flute. However, the chip does not have an inherent tendency to move axially, but has to be pressed by compulsion through the chip flute. The so-called start chip—generated by, for instance, a pointed, symmetrical twist drill, the chip flutes of which are helicoidal—that is, the front part of the chip initially generated upon engagement or entering, is reasonably narrow, whereupon the width of the chip grows until the cutting edges attain full engagement. This full chip obtains a helicoidal or hair-lock-like shape, the diameter of which is determined by the cross-section area of the flute space that is delimited by the hole wall as well as by the concave limiting surface of the chip flute. Just the start chip has no natural tendency to move axially. On the contrary, the same tends to be pressed out radially and deflect against the hole wall, which causes poor chip control.
Different embodiments of chip flutes of drills are disclosed in, for instance, U.S. Pat. No. 6,988,859 B2, U.S. Patent Application Publication No. 2003/0175086 A1, and Swedish Patent No. 528156 C2, which show symmetrical twist drills having two cutting edges and two helicoidal chip flutes. Common to these previously known drills is that the individual chip flutes run with one and the same generally concave cross-section shape up to the front end surface into which they transform via smoothly rounded borderlines. The concave cross-section shape is primarily determined by the profile shape of the grinding disc used to provide the chip flute. The grinding of the chip flutes aims at allowing the chip to slide with the peripheral, helicoidal outer edge thereof along the ground surface as frictionless as possible. In this connection, the chip flutes of certain types of drills are even polished to decrease the frictional resistance. However, a disadvantage of the previously known drills is that they present a mediocre, initial chip control so far that the narrow start chip tends to be pressed out toward the hole wall, i.e., move radially rather than axially. It may even happen that the chip is wedged in between the hole wall and the cylindrical envelope surface of the drill body.
The present invention aims at obviating the above-mentioned disadvantage of previously known drill bodies and at providing an improved drill body.
An object of the invention to provide a drill body that ensures a good, initial chip control and therefore reduces the risk of the emergence of disturbing phenomena, such as chip jamming or the start chip penetrating in between the hole wall and the envelope surface of the drill body.